Oligonucleotides such as DNA and RNA have been considered to mainly have functions as molecular species involved mainly in protein synthesis, and have attracted attention since phenomena such as in a ribozyme or RNAi, in which nucleic acid molecules can interact directly with molecular species such as proteins or macromolecules, thereby regulating functions of such molecular species, have been discovered. In particular, an aptamer has recently attracted attention as a nucleic acid capable of binding directly to a molecular species such as a protein, thereby modifying the function of the protein. A large number of novel aptamers have been obtained for the purpose of applying them to a pharmaceutical or the like.
On the other hand, an antibody belonging to a class such as IgG, which is derived from a laboratory animal such as a mouse, rat or rabbit, collectively refers to a proteinous substance capable of binding, in a binding mode such as hydrogen bond, to a high-molecular compound such as a protein or a antibody capable of forming an antigen-antibody complex. Such an antibody is widely used in the field of medicine (for example, as a diagnostic agent) as an antibody that specifically binds to, for example, an antibody in an antigen-antibody complex.
However, although such an antibody is useful in that it has a specific activity for the counterpart antigen, various problems have been mentioned in preparation of the antibody. For example, in order to prepare antibodies, after an antigen is repeatedly injected into an animal to be immunized such as a mouse, rat or rabbit, thereby inducing the immune-reaction, it is required to obtain a desired fraction having an ability to bind to the antigen from serum etc. Therefore, such a process of preparation is very disadvantageous in terms of laborsaving and costs. Moreover, the antibody has nonspecific binding properties to various proteins or polypropylene or polyethylene containers other than the antigen that the antibody specifically binds to, and is also disadvantageous in terms of ease in handling. Furthermore, preparation of antibodies requires use of immune animals as described above, and such use of immune animals is not preferable from the viewpoint of animal protection.
When an antibody is used as a secondary antibody for the above-mentioned diagnostic agent, the antibody needs to be prepared as a complex with a labeling compound such as a peroxidase in order to spectrophotometrically detect the degree of binding to an antigen-antibody complex. However, preparation of such a complex further makes the process complex besides preparation of the secondary antibody.
Accordingly, a molecular species capable of binding specifically to an antigen has been sought as an alternative to an antibody.    Non-Patent Document 1: M. Zuker, Science, vol. 244, pp 48-52, 1989